The present invention relates to an electric discharge machining apparatus and, more particularly, to an apparatus for detecting a discharge gap in the electric discharge apparatus.
In electric discharge machining, an intermittent DC voltage in the form of a pulse voltage is supplied between a machining electrode and a workpiece to thereby produce an electric discharge or an arc which is used to machine the workpiece to a desired depth and configuration. During the machining, the appearance of the electric discharge varies delicately in response to changes in the discharge gap between the machining electrode and the workpiece, even though the supplied DC voltage remains constant. As a result, the surface finish of the workpiece and the machining speed are undesirably varied.
For this reason, in actual machining, in order to obtain a uniform machining results and a desired machining speed, servocontrol is employed in which the discharge gap is measured by detecting a discharge voltage, and either the workpiece or the machining electrode is delicately controlled such that a desired discharge gap is maintained. One example of this kind of the servocontrol is shown in U.S. Pat. No. 4,339,650 to Tanaka et al, "ELECTRIC DISCHARGE MACHINING APPARATUS".
To measure the discharge gap, it is known to detect a discharge initiating voltage (peak voltage) or an average value of the discharge voltage which is measured at the machining electrode. By such conventional detecting methods, however, it is not possible to effect accurate detection over a wide gap range. For instance, with the conventional method which employs the peak voltage, which is always equal to the value of the supplied pulse voltage itself in an area where the discharge gap is wide, that is, in a case where discharge occurs after a certain period of time elapses after the supply of the pulse voltage, it is not possible to detect any change in the actual discharge gap. Also with the method which employs the average value, on the other hand, although it is possible to effect approximate detection in the area where the discharge gap is wide, it is not possible to accurately detect the discharge gap where the discharge gap is narrow, that is, in a case where discharge occurs immediately after the supply of the pulse voltage.